In general, a heliostat used in a tower-type solar thermal collection system includes a reflecting mirror that reflects sunlight, a supporting post that supports the reflecting mirror, and a drive device that tilts the reflecting mirror. In order to concentrate sunlight on at receiver installed in the tower, the orientation (azimuth angle and elevation angle) of the heliostat is adjusted on the basis of a control signal related to the tilt angle of the reflecting mirror.
A basic principle for controlling the orientation of the heliostat will be described by using FIGS. 10 and 11. FIG. 10 illustrates a relationship among the azimuth angle, the elevation angle, and a normal vector of the reflecting mirror. A general heliostat has a configuration in which at least one angle of the azimuth angle and the elevation angle (in most cases, both the azimuth angle and the elevation angle) of the normal vector of the reflecting mirror can be changed to any angle. Thus, as illustrated in FIG. 10, if the normal vector of the reflecting mirror is known, the azimuth angle and the elevation angle of the heliostat for concentrating sunlight on the receiver can be calculated. The normal vector of the reflecting mirror can be calculated on the basis of the law of reflection if an incident light vector and a reflected light vector are known. In description provided below, the azimuth angle and the elevation angle may be collectively referred to as “controlled angle”.
FIG. 11 illustrates a calculation method for the incident light vector and the reflected light vector. First, the azimuth angle and the elevation angle of the sun can be calculated on the basis of longitude and latitude information at a position where the heliostat is installed and date and time information at any point in time (in a time period in which the heliostat is desired to be controlled). Thus, the incident light vector can be obtained by converting the azimuth angle and the elevation angle of the son into a vector. Next, the three-dimensional coordinates of the position of the heliostat (center of the reflecting mirror=starting point of the normal vector of the mirror) and the three-dimensional coordinates of an aiming point (for example, a light receiving surface of the receiver) which is a target of the reflected light are previously known at a stage where the device is drawn in a drawing or at a stage where the device is installed. Thus, the reflected light vector can be calculated from a positional relationship between these sets of three-dimensional coordinates.
Hereinafter, the controlled angle (azimuth angle and elevation angle) of the heliostat calculated from the normal vector obtained on the basis of the incident light vector and the reflected light vector, that is, the controlled angle of the heliostat calculated from the above basic principle, will foe referred to as “theoretical value”. Meanwhile, the controlled angle (azimuth angle and elevation angle) of the heliostat when the reflected light is actually concentrated at the aiming point will be referred to as “measured value”.
FIG. 12 is a graph comparing the measured value and the theoretical value according to time. As understood from FIG. 12, a deviation of, for example, +1.5° or less is generated between the measured value and the theoretical value in one day by a number of various factors such as error upon manufacturing the heliostat and error in the position where the heliostat is installed. If the deviation can be reduced to, for example, +0.2° or less by installing the heliostat with high accuracy or by measuring installation error, the heliostat, theoretically, can irradiate the light receiving surface of the receiver with the reflected light at all times. However, a solution that uses the accuracy of installation of the heliostat and the like to absorb the deviation results in an enormous amount of construction time and enormous construction cost and thus may not be a realistic solution.
A technology that reduces the deviation by performing calibration of the heliostat after installation is known (for example, refer to PTL 1). In PTL 1, the reflected light upon performing calibration is concentrated at a plurality of aiming points such as cameras disposed in a field, and the position of the heliostat to be set is estimated from the deviation between the measured value and the theoretical value at that point in time. According to PTL 1, since high accuracy installation or measurement of installation error is not required, the amount of construction time and construction cost can be reduced.